Estimation on the refractive index of ambient aerosols as fine- and coarse-mode volume size distribution

The refractive index (RI) of aerosol is an important parameter that reflects the scattering and absorption capacity of aerosol and is widely used in atmospheric models and remote sensing studies. It depends on several properties such as chemical species, moisture content, etc. RI is usually obtained by the chemical method using a volume mixing ratio of known chemicals and by the optical method based on extinction, scattering and/or absorption coefficient using Mie theory. However, these methods are complicated and are mainly for singular particles. In this study, we tried to make a simple method to estimate RI by measuring the extinction coefficient from camera images and size distribution from the optical particle counter (OPC). We used the wavelength of Red-Green-Blue color as 597, 534, and 459 nm to calculate the extinction coefficient and the number size distribution to retrieve the volume size distribution obtained by OPC. The volume size distribution is expressed by six parameters of two gaussian graphs for fine and coarse-mode particles. We got the volume, median radius, and standard deviation of fine and coarse aerosol peaks. The measurement site was the port area in Busan, Korea. We sometimes discovered the emission of particles but mainly measured for clear days. We effectively retrieved six parameters determining the volume size distribution of ambient aerosols and tried to inverse RI as total, fine, and coarse particles using the nonlinear optimization method. We mainly considered the scattering effects of ambient aerosols, so we focused on the real part of RI. The retrieved RI was from 1.27 to 1.50 and showed different values on fine- and coarse-mode size particles. The results were lower than other RI measurement studies, so we need to validate more. However, we identified the possibility of retrieving the RI of aerosols using the known size distribution retrieved from OPC and the three-color extinctions from the camera.